Recently the importance of lithium-ion batteries, nickel-metal hydride batteries, and other secondary batteries as a power source not only for mounting in vehicles, but also for personal computers and other portable devices, has increased. In particular, lithium-ion batteries that are lightweight and provide a high energy density are expected to be used most suitably as high output power sources to be mounted in vehicles. In these types of batteries a battery structure is known that comprises a wound electrode assembly wherein a sheet-shaped positive electrode and a sheet-shaped negative electrode are laminated and wound together with a separator interposed therebetween. For example, Patent Document 1 discloses a battery comprising an electrode assembly wherein a positive electrode sheet and a negative electrode sheet are wound together with a separator interposed therebetween.    Patent Document 1: Japanese Patent Application Laid-open No. 2003-249423
In these types of batteries, the electrode assembly and the exterior case must be manufactured separately, and then the electrode assembly must be housed in the exterior case. A metal case is used as the exterior case because it has great physical strength, and in such a case a process of wrapping the electrode assembly in an insulating film to insulate the metal case and the electrode assembly is typically carried out. FIG. 8 shows one example of a process for a battery that includes such a wrapping process. The battery is constructed by first housing (packaging) the electrode assembly 1 in a box-shaped insulating film 2, then housing the insulating film 2 together with the electrode assembly 1 into the exterior case 3, and finally closing the top opening of the exterior case 3 with a lid (not illustrated).
However, there has been a problem because in conventional battery construction the insulating film is in the shape of a box having corners, and that makes it difficult to insert the insulating film (and electrode assembly) into the exterior case. For example, as shown in FIG. 9, when the inside four corners (corner members) 3b of the exterior case 3 have a rounded shape, the corners 2a of the insulating film 2 can interfere with the corner members (in this case rounded members) 3b of the exterior case 3, resulting in detrimental effects on insertability, and there is concern that in some cases creases, kinks and gaps in the insulating film 2 can occur. If the final product is manufactured with creases in the insulating film 2, the restraining force from the outside will be applied as an uneven tension on the interior of the battery. This is undesirable because there is a risk that this will have a locally detrimental effect on electrolyte circulation.